Mass spectrometer



Aug. 30, 1960 R. L. KINDRED MASS SPECTROMETER 2 Sheets-Sheet 1 FiledJuly l0, 1957 .v ll. mm1...

ATTORNEYS v`INVENTOR.

R. L. KIN DRED Aug. 30, 1960 R. L. KINDRED MAss SPECTROMETER 2Sheets-Sheet 2 Filed July lO, 1957 INVENTOR. R. KINDRED HMM* @ON @ON.SNN MN mmDF N mFmEOmPUmn-w DOQ ATTORNEYS nited States Patent;

MASS SPECTRONLETER Raymond L. Kindred, Bartlesville, Okla., assgnor toPhillips Petroleum Company, a corporation of Dela- Ware Filed July 10,1957, Ser. N0. 671,079

7 Claims. (Cl. Z50-441.9)

This invention relates to a mass spectrometer which is adapted tomeasure a plurality of constituents of a sample mixture.

In recent years mass spectrometers have been developed from highlyspecialized academic research instruments for measuring the relativeabundance of isotopes into analytical tools of extreme sensitivity andaccuracy. At the present time, applications are being found for the useof mass spectrometers in process monitoring and control. Massspectrometry comprises, in general, ionizing a sample of material underinvestigation and separating the resulting ions according to theirmasses to determine the relative abundance of ions of selected masses.'I'he material to be analyzed usually is provided -as a gas, which isbombarded by a stream of electrons to produce the desired ions. Althoughboth positive and negative ions may be formed by such electricalbombardment, most mass spectrometers make use of only the positive ions.These positive ions are accelerated out of the region of the electronbeam by means of negative potentials. Such potentials impart equalkinetic energies to ions having like charges such that ions of differentmasses have different velocities after passing `through the electricaleld and, consequently, have different momenta.

The presently known mass spectrometers can be classied in one of twogeneral groups: the momentum selection types and the velocity or energyselection types. The momentum selection instruments sort the ions intobeams of diiferent masses by the use of magnetic and/or electricaldeectingiields. Ions of selected masses are lallowed to impinge upon a-collector plate to which is connected an indicating circuit. Thevelocity or energy selection instruments sort the ions according to thevelocities or energies imparted to ions of selected masses by electricalaccelerating elds. The present invention is particularly adapted to beemployed with a mass spectrometer of the latter type, `although it isnot limited thereto.

In accordance with this invention, a sample system is provided to directa plurality of fluid streams to be analyzed .into theV mass spectrometertube in sequence. Means also are .provided for introducing astandardiza- Ition gas into the tube periodically to check the operationof the mass spectrometer itself. The ows of the several streams into thespectrometer tube are controlled automatically by a timing circuit.During the time that any given sample stream is owing into thespectrometer, the operating potentials applied to the spectrometer tubeare adjusted in sequence so that the tube -is focused to detect theindividual constituents of the iluid mixture to be analyzed. Means alsoare provided to adjust the accelerating potential so as to scan each ofthe mass peaks.

Accordingly, it is an object of this invention to provide an improvedmass spectrometer which is adapted to measure :a plurality ofconstituents of a mixture.

Another object is to provide a sample systemfor a mass spectrometerwherein a plurality of uid'streams K 2,951,155 Patented Aug. 30, 1960Vto be measured and a standardization lgas are directed Other objects,advantages `and features of the invenytion should become apparent fromthe following detailed description which is taken in conjunction withthe accompanying drawing inwhich:

Figure 1 is a schematic representation of -a mass spectrometer having aportion of the potential adjusting mechyanism of this inventionassociated therewith.

Figure 2 is a schematic representation of the sample system and timingmechanisms which are employed in conjunction with the mass spectrometertube of Figure 1. Y

Referring lnow to th..- drawing in detail and to Figure l in particular,there is shown a mass spectrometer tube 10 which comprises a gasimpermeable envelope, the interior of which is maintained ata reducedpressure by a vacuum pump, not shown, which communicates with theinterior of tube 10 through a conduit 11. A sample of gas to be analyzedis directed into tube 10 through a conduit 12. An electron emittinglament 13 lwinding 17 of transformer 16 are connected to an alternatingcurrent source 18. The center tap of the secondary winding 15 oftransformer 16 is connected -to a `negative potential terminal 19.

The gas sample supplied through conduit 12 is directed into anionization chamber 22 in tube 10 which is defined by 'a pair of spacedgrids 23 land 24 Ythat are maintained at ground potential. Electronsemitted :from lilarnent 13 :are accelerated into chamber 2.2 by thepotential d-itference between filament 13 Eand grids 23 and 24. A -grid26 is positioned between filament 13 and lgrid 23. Grid 26 is connectedto the output of an emission regulator circuit 27, which can be of thetype disclosed vin the copending application of M. C. Burk, Serial No.412,790, liled February 26, 1954, now U.S. Patent No. 2,792,500, grantedMay 14, `1957. The input of emission regulator 27 is connected to thecenter tap of transformer winding 15. This emission regulator isprovided for the purpose of applying `a potential to grid 26 ofmagnitude such as to maintain .a constant flow of electrons intoionization chamber 22, irrespective of minor fluctuations in theelectron .emission from lament 13. In this manner the rate at which gasmolecules are ionized in chamber 22 by electron bombardment is afunction of only the gas pressure in the chamber.

A first collim-ating electrode 28 is positioned on the second side ofionization chamber 22 and islconnected to the contactor of apotentiometer 20. One end terminal of potentiometer 2.0 is connected toa negative potential terminal 21, the lsecond end terminal ofpotentiometer 20 being grounded. A second collimating electrode 28 isspaced from electrode 28, Electrode 28 is connected to the contactor ofa potentiometer 20. The end terminals of potentiometer 20' are connectedto terminal 21 and ground, respectively.

The positive ions produced in ionization chamber 22 are accelerated bythe negative potentials applied to electrodes 28 and 28 so as to travelthrough the tube toward collector plate 14. A first set of three equallyspaced grids 35, 36 and 37 are positioned in tube 10 between lgrid 28:and collector plate 14. A second set of equally spaced grids 38, `39and 40 are positioned in spaced relation with the iirst set of grids; rathird set of equally spaced grids 42, 43, and 44 are positioned inspaced relation with the second set of grids;'a fourth set of -in spacedrelation with the fourth set of grids. The

spacings s between grids 35 and 36, 36 and 37, 38 and '39, 39 and 40, 42and 43, 43 and 44, 45 and 46, 46 and 47, 49 and 50, and S0 and 51 areequal. The spacings r'lbetween the centers of grids 37 and 38, 40 and42,

44 and 45, and 47 and 49v can be represented by the exwhere n is anintegral number.

. Grid 35 is connected to a switch 60 which is actuated by a relay 61a.Switch 60 engages a terminal 62 when relay 61a Ais de-energized and aterminal 63 when relay 61a is energized. Terminals 62 and 463, areconnected to the contactors of respective potentiometers 64 and 65. Therst end terminals of potentiometers 64 and 65 are connected to groundthrough a variable resistor 66. The second end terminalsv ofpotentiometers 64 and 65 are connectedto respective terminals 67 and68'. A switch 69 engages terminal 67 when a relay 61h is de-energizedand a terminal 68 when relay 61b is energized. Switch 69 is connected toa terminal 70 which is maintained at a potential which is negative withrespect to ground. Variable resistor 66 is adjusted by a constant speedmotor 72 which is energized by a current source 73. Motor 72 adjuststhis resistor so that the resistance thereof continually increases, thenis decreased, and repeats at a predetermined frequency.

Grid 51 is connected to a switch 75 which is actuated by a relay. 76a.Switch 75 engages a terminal 77 when relay 76a is de-energized and aterminal 7'8 when relay 76a is energized. Terminals 77 and 78 areconnected to the contactors of respective potentiometers 80 and 81. VTherst terminals of potentiometers 80 and 81 are connected to grid 35 andto a terminal 82. The second end terminals of potentiometers 80 and -851are connected to respective terminals `83 and 84. A switch 85 engagesterminal 83 when a relay 76b is de-energized and a terminal 84'wl1enrelay 76b is-energized. Switch V85 is connected to a terminal 87. Asource of direct potential, not shown, Vis connected between terminals82 and 87 so that terminal 82 is negative with respect to terminal 87.Grid `51 is connected to grids 47 and 49 through a resistor 88;

grids 47 and49 are connected to grids 414 and 45 through a resistor 89;grids 44 and 45 are connected to grids 40 and 42 through a resistor 90;grids 40 andv42 are connected to grids 37 and 38 through a resistor v91;and grids 37 and 39 are connected to grid 35 through a resistor 92.

Grids 36, 39, 43, -y46 and 50 are connected to one another and to thefirst output terminal of an oscillator 95, the second output terminal ofwhich is connected to ground. The frequency of oscillator 95? isdetermined by a crystal 96 or 97 which is connected in the oscillatorcircuit. The rst terminals of these crystals are connected to a commonterminaly and oscillator 95. The second terminals of crystals 96 and 97are connected to respective terminals 9S and 99. A switch .100 engagesterminal 99 when a relay 101 is de-energized and a terminal 9S whenrelay 1 01 is energized. Switch 100 is connected to the second terminalin oscillator 95 to complete the circuit with one of the crystals. Ifthe masses of the dilerent ions to be detected are nearly the same, onlyone output frequency from oscillator 95 normally is provided. Thedifferent frequencies are useful in focusing the tube when the masses ofthe ions are substantially different.

A plurality of closely spaced stopping grids 105 are positioned betweengrid 51 and collector plate14. Grids =105 are. connected, to one'another' andto a switchlll which is actuated by a relay 76C. Switch :106engages a terminal 107 when relay 76e is de-energized and a terminal 108when relay 76C is energized. Terminals 107 and are connected to thecontactors of respective potentiometers 109 and 1.10. The first endterminals of potentiometers 1109 and 110 are connected to ground. Thesecond end terminals of potentiometers v109 and 1110 are connected torespective terminals 111 and 1.12. A switch 113 engages terminal 111when a relay 76d is de-energized and a terminal 11x12 when relay 76d isenergized. Switch 113 is connected to a terminal l114 which ismaintained at a potential which is positive with respect to ground.

A plurality of suppressor grids .120 are positioned between grids 105and collector plate 14. Grids 120 are connected to a terminal 121 whichis maintained at a potential which is negative with respect to ground sothat grids suppress secondary electrons which may result from ionsimpinging upon metal parts ofthe tube. A grounded shield 122 ispositioned adjacent collector vplate 14.

Collector plate :14 is connected to the first input terminal of adetector circuit `124, the second input terminal vofwhich is grounded.The output terminals of detector circuit 124 are connected through aswitch network 125, which is described hereinafter, to a recorder 126.

The positive ions formed in chamber 22 are accelerated toward collectorplate 14 by the negative potentials applied to grids 28 and 2 8. Duringone half cycle of the output signal from oscillator 95,V the electricalfield between grids 35 and 36 is of such phase that the ions enteringthis eld are accelerated. Ions which enter the field during a particularphase of this half cycle receive maximum energy. During the followinghalf cycle of the signalA from oscillator 95, the field between grids 36and 37 is such that the ions are further accelerated. These ions thendrift through the field-free space between grids 37 and 38. The massesof the individual ions determine their times of arrival at grid 38. Theions which arrive at grid 3S at the proper time are again accelerated amaximum Vamount by the field applied between grids 38 and 39, therebyreceiving additional energy. The same accelerat- Ving procedurecontinues as the ions pass through the next ten grids. The positivepotential applied to grids 105 is adjusted such that only thoseionswhich receive a predetermined maximumV energy are able to passthrough grids 105 to impinge upon collector plate 14.

The ions impinging upon collector plate 14 cause current to fiow in theinput circuit of detector 124.` This current is ypro} oortional inmagnitude to the number of ions impinging upon the collector plate. Thepotential applied across the resistance network defined by resistors 88,89, 90, 91 and 92 serves to decelerate the ions so that the velocity ofthe selected ions to which the spectrometer tube is tuned remainsubstantially constant, whereas the ions of other masses aredeceleratedin passing through the tube. For a more detailed descriptionof the operation of the mass spectrometer tube thus far described,including oscillator 95, reference is made to U.S. Patent`2,7 61,974.

InY order to describe'the mass spectrometer of this invention, referencewill be made to a particular system wherein three sample streams areydirected to the spectrometer tube in sequence. It should be evident,however, that an additional number of sample streams can readily beprovided, The gas samples to beV analyzed are introducedV into thesystem by means of conduits x130, .131 and 132 of FigureZ which haverespective solenoid operated valves 133, 134 and 135 therein. Ventconduits 130:1, 13 1a and 132a communicatewith respective conduits 130,131 and 132upstream from valves 1133', -134.and 135. Conduits 130, 131and 13,2 communicate with a common Y conduit .136 which has apressureregulator 137 therein. :naluit 13.6 sommvnistefs, Withths firstetsf .a three aesinet# way solenoid operated valve 138. Standardizationgas for the mass spectrometer is supplied by a tank 140, the outlet ofwhich is connected by a conduit 1'41, which has a pressure regulator 142therein, to the second inlet valve 138. The outlet of valve 138communicates with a conduit y143 which has a `ilow controller 144therein. 'A vent conduit 145 communicates with conduit 1-43 so that thesample to be analyzed flows continuously through conduit 143. A conduit146, having a restricted passage 147 therein, communicates with conduit.143. A conduit 148 communicates between the second end restrictedpassage 147 and the inlet of a Vacuum pump 149. A small volume of thesample thus flows through passage 147. A second restricted passage 150communicates between conduit 146 and conduit '12. Conduit 1&1 isconnected to the inlet of vacuum pump 151. A sample of the material tobe analyzed is thus directed through spectrometer tube from passage 150.l

The solenoids 133,', 134', 135 and 138' which actuate respective valves133, 134, -135 and 138'are energized in sequence by a timing circuit.-This timing circuit is controlled by a constant speed timing motor 160which is connected across a source of alternating current 161. Motor 160closes a switch 162 momentarily at periodic intervals. This can beaccomplished, for example, by means of a `cam connected to the driveshaft of motor 160. The tirst terminal of current source 161 isconnected to the iirst terminal of a rectifier 163. The second terminalof rectifier 163 is connected through a capacitor 164 to the secondterminal f161b of current source 161. A D.C. voltage thus appears acrosscapacitor 164. This voltage is applied through switch 162 to a coil 165of a stepping switch. When the coil 165 is energized, arm 166 of thestepping switch moves from one to the next contact 167a, 167b, 167C,167d and 167a. Switch arm 166 is connected to terminal 161b. Terminals167:1, 167b, 167C and 167d are connected to terminal 161g through thecoils of respetcive relays 168:1, 168b, 168e and 168d. The primarywinding 170 of a transformer 1711 is connected across current source161.A The first terminal of the secondary winding 172 of transformer 171is connected to switches 173:1, 173b, 173C and 173d which are closedwhen respective relays 168:1, 1681;, 168e and 168:1 are energized. Thestationary terminals of switches 173:1, '17311, 173C and 173:1 areconnected to the first terminals of respective solenoids 133', 134',135' and 138. The second terminals of solenoids 133', 134', 135' and 138are connected through a resistor 174 to the second end terminal oftransformer winding 172.

"Each time switch 162 is closed by timing motor 160, switch cam 166moves to the next terminal. This energizes relays 168:1, 168b, 168C and168d in sequence so that valves 133, 134, 135 and 138 are actuated insequence. Conduit 136 communicates with conduit 143 except when solenoid138 is energized. The three sample streams and the standardization gasare thus directed to the spectrometer tube in sequence.

The coil of relay 180 is connected in parallel with resistor 174. Aswitch 181 engages a terminal 182 when relay 180 is energized. Switch181 is connected to terminal 161:1, and terminal 182 is connected to aswitch 183. Switch 183 engages a terminal 184 when a relay 185 isde-energized and a terminal 186 when relay 185 is energized. Terminal184 is connected tov a switch 187 which is adapted to engage either aterminal 188 or a terminal 189. Switch 187 normally engages terminal188. Terminal 188 is connected to the rst terminal of a timer 190. Thesecond terminal of timer 190 is connected to terminal 161b. Timer 190moves switch 187 into engagement with terminal 189 a predetermined timeafter being energized. Terminal 189 is connected to terminal 186 and toone terminal of the coil of relay 185. The second terminal of the coilof relay 185 is connected to terminal 161b. A capacitor 191 is connectedin parallel with the coil of relay 185.

Relay 185' alsoactuates a second switch 192 which engagesl a terminal193 when the relay is energized. Switch 192A is connected to terminal161a, and terminal 193 is connected to terminal 161b through the `coilof relay 101 of Figure 1 to terminal 161b. Relay 185 is provided with athird switch 195 which engages a terminal 196 when the relay isde-energized and a terminal 197 when the relay is energized. Terminal197 is connected to terminal 186, and terminal 196 is connected toterminal 188. Switch 195 is connected to a switch 200 which isactuated'by ayrelay 201. Switch 200 engages a terminal 202 when relay201 is de-energzed and a terminal 203 when'the relay is energized.Terminal 202 is connected to a switch 204 which normally energizes aterminal 205. Terminal 205 is connected to the first terminal Yof asecond timer 206 which moves switch 204 into engagement with a terminal207 a predetermined time after the timer is energized. The send terminalof timer 206 is connected to terminal 161b. Terminals 203 and 207 areconnected to the yfirst terminal of the coil of relay 201, the secondterminal of which is connected to terminal 161b. A capacitor 208 isconnected in parallel to the coil of relay 201. The first terminal, ofthe coil of relays 61, which corresponds to relays 61a and 61b of Figurel, is connected to terminal 161b. The second terminal of relay coil 61is connected to terminals 186 and 197. VA coil 76, which corresponds torelays 76:1, 76b, 76C and 76d of Figure l, is connected in parallel withcoil 61.

It can be seen that relay 180 is energized each time one of the solenoidactuated valves in the sample lines is energized. The closure of switch181 results in timer 190 being actuated. At the end of the predeterminedtime interval, timer 190 moves switch 187 into engagement with terminal189.- This energizes relay 185. Relays 101, 61 and 76 are energized atthis time so that potentiometers 65, 81 and 110 are connected into thecircuit of Figure 1 in place of potentiometers 64, 80 and 109. `Crystal96 is also connected into the circuit in place of crystal 97. Themassspectrometer initially is adjusted by means of potentiometers 64, and109 so that the instrument detects one of the constituents of the fluidmixture to be analyzed. Timer 190 serves to change the potentialsapplied to the spectrometer tube and the frequency of oscillator so thatthe instrument detects a second constituent in the fluid sample to beanalyzed.

Closure of switch 181 also energizes timer 206. At the end of apredetermined interval, timer 206 moves switch 204 into engagement withterminal 207. This energizes relay 201 to move a switch 210 intoengagement with a terminal 211. Closure of switch 210 connects the coil212 of the pen-drive motor in recorder 126 into the circuit. A record isthus made only after a predetermined time interval following theswitching operation. This permits the mass spectrometer to becomestabilized before a record of the constituent being analyzed is made.

The recorder can advantageously be provided with two separate recordingpens to provide individual records of the two constituents to bemeasured. These two pens are actuated by respective solenoids 214 and215 being energized. The rst terminals of these two solenoids areconnected to ground. The second terminals of' solenoids 214 and 215 areconnected to respective terminals 216 and 217 which are adapted to beengaged by a switch 218, the latter being actuated by relay 185. Powerfor relays 214 and 215 is obtained from a bridge rectifier 241 which isconnected across the secondary winding of a transformer 240. The primarywinding of transformer 240 is connected across current source 161. Theoutput Voltage of rectifier 241 is filtered by a circuit comprising aresistor 242 and capacitors 243 and 244. Solenoid 214 is thus energizedduring the first half of the analysis cycle and solenoid 215 isenergized during the last-half of the analysis cycle. v

Switch network V125 isadapted short the input teron the record. The twooutput `terminals of detector 124 are connected to respective terminals220 and 221 which are adaped to be engaged by respective switches 222and 223 when a relay 224 is de-energized. Switches 222 and 223 areconnected to the respective input terminals of recorder 126. Switches222 and 223 engage respective terminals 225 and 226 when relay 224 isenergized. Terminals 225 and 226 are connected to one another to shortrecorder 126. One terminal vof the coil of relay 22e is connected toterminal 161b. The second terminal of this relay coil is connectedthrough a resistor 230 and a switch 231 of athermal timedelay switch 229to switch 1736!. A heating element 232 and a resistor 233 are connectedin series -between switch 173d and terminal 161b. When relay 168d isenergized to begin the standardization cycle, current is supplied toheating element 232. After'a predetermined interval, this heating servesto open thermal switch 231 so that relay 224 is de-energized. Prior tothis time, the input terminals of record 126 are shorted.

As an example of the operation of the mass spectrometer of thisinvention, one instrument has been employed successfully in the analysisof converter recycle streams in the manufacture of ammonia. Thesestreams generally contain approximately 60 percent hydrogen, 20 percentnitrogen, 10 percent methane, and l0 percent of other gases. Theanalyzer timer was set to pass each sample stream to the spectrometertube for approximately six minutes, giving two half cycles of 3 minuteseach. Timer 190 provides the two half cycles of 3 minutes each oneach'stream. Timer 206 is provided with a delay of 2 minutes to permitthe analyzer to become stabilized after the switching. This gaveapproximately one minute each for the analysis of hydrogen and nitrogenin each sample stream. The potentials and frequencies applied to tube 10were adjusted to permit detection of these constituents.

From the foregoing description it should be evident that there isprovided in accordance with this invention an improved mass spectrometerwhich is adapted to analyze a plurality of sample stream periodicallyand to detect individual constituents ofthe streams. VWhile theinvention has been described in conjunction with theV analysis of iiuidstreams to detect two constituents thereof, it should be evident thatthe switching system v can be extended to the analysis of additionalconstituents in sample streams. Provision is also made in accordancewith this invention for Calibrating the analyzer periodically by meansof a standardization gas of predetermined composition.

While the invention has been described in conjunction with a presentpreferred embodiment, it should be evident that it is not limitedthereto.

What is claimed is:

l. A mass spectrometer comprising a gas impermeable envelope enclosingan ionization chamber, a collector plate spaced from said ionizationchamber, a plurality of groups of grids spaced in a line betweensaidionization source and said plate, each of said lgroups comprisingthree grids in spaced relationship with one another, the spacingsbetween adjacent grids being equal, the spacings between adjacent groupsof said grids being substantially 7L zsimal) where n is an integralVnumber and s is the spacing between adjacent grids in each group, andan ion stopping grid positioned between said collector plate and saidgroups of grids; first and second sources of negative potential; lfirst.switching means to apply said iirst and second sources of negativepotential selectively to the 'grid-Qt Said plurality .0f .groups ofigridsclosest .to Asaid .tween adjacent grids in each group,

ionization chamber; apotential dividing network; means to connect theend grids ofsaid plurality of groups of grids tosaid network; third andfourth sources of direct potential; second switching means to apply saidthird .and fourth sources of potential selectively across said network;fth and sixth sources of positive potential; third switching means toapply said fifth and sixth sources of lpotential to said ion stoppinggrid; means to apply an alternating potential to the center grids ofsaid groups of grids; `detector circuit means connected to saidcollector plate; means to introduce a plurality of sample streams to beanalyzed into saidionization chamber in sequence; and timing means toactuate said means to introduce and saidswitching means so as to applysaid iirst, third and fifth sources of potential tothe respective gridsfor a predetermined time and thereafter to apply said second, fourth andsixth sources of potential to the respective grids each time a differentstream is passed to said ionization chamber.

2. The mass spectrometer of claim 1 further comprising means to varyprogressively the amplitude of the source of negative potential which isapplied to the grid of said plurality of groups of grids closest to saidionization chamber.

3. The mass spectrometer of claim l wherein said de tector circuit meansincludes a recorder having two recording means therein to make twoseparate records, and further comprising means responsive to said timingmeans to energize one of said recording means when said first, third andfifth sources of potential are applied and to energize the other of saidrecording means when said second, fourth and sixth sources of potentialare applied.

4. The mass spectrometer of claim 1 wherein said detector circuit meansincludes a recorder, and means to render said recorder inoperative for apredetermined interval after said iirst, third and iifth sources ofpotential are applied and to render said recorder inoperative for apredetermined interval after said second, fourth and sixth sources ofpotential are applied.

5. The mass spectrometer of claim 1 wherein said means to introduce aplurality of sample streams comprises a plurality of first conduit meanshaving respective first control valves therein, second conduit meanscommunicating at one end withsaidV plurality of iirst conduit meansdownstream from said control valves, third conduit means communicatingwith the second end of said second conduit means to vent `fluid, fourthconduit means having a iirst restriction therein, one end of said fourthconduit means communicating with the second end of said second conduitmeans, a iirst vacuum pump having the inlet thereof communicating withthe second end of said fourth conduit means, fifth conduit means havinga second restriction therein communicating between the second end ofsaid fourth conduit means and the ionization chamber in said envelope,and a second vacuum pump having the inlet thereof communicating with theinterior of said envelope.

6. A mass spectrometer comprising a gas impermeable envelope enclosingan ionization chamber, a collector plate spaced from said ionizationchamber, a plurality of groups of rst grids spaced in a `line betweensaid ionization chamber and said plate, each of said groups comprisingthree grids in spaced relationship with one another, the spacingsbetween adjacent grids being equal, the spacings between adjacent groupsof grids being substantially where fz is an integral number and s is thespacing begrid positioned between said collector plate and said groupsof grids; rst and second sources of negative potential; iii-stswitching-means to apply `said first and second sources tof negativepotential selectivelysto the Agrid and an ion stopping of said pluralityof groups of grids closest to said ionization chamber; a potentialdividing network; means to connect the end grids of said plurality ofgroups of grids to said network; rst and second sources of directpotential; second switching means to apply said first and sec- `ondsources of direct potential selectively `across said network; rst andsecond sources of positive potential; third switching means to connectsaid lirst and second sources of positive potential to said ion stoppinggrid; means to apply an alternating potential to the center grids ofsaid groups of grids comprising a source of alternating potential andmeans to vary the frequency of said source of alternating potential toestablish first and second alternating signals of ditferent frequencies;detector circuit means connected to said collector plate; and timingmeans to actuate said switching means to apply said first source ofnegative potential, said rst source of direct potenti-al, said firstalternating signal, and said first source of positive potential to therespective grids for a predetermined time and thereafter to apply saidsecond source of negative potential, said second source of directpotential, said second alternating signal, and said second source ofpositive potential to the respective grids.

7. A mass spectrometer comprising an ionization chamber; a collectorplate spaced from said ionization chamber; a grid positioned betweensaid ionization chamber and said collector plate; first and secondsources of potential of polarity opposite the polarity of the ions to bemeasured; switching means to apply said irst and second sourcesselectively to said grid to accelerate ions from said ionization chambertoward said collector plate; means to focus ions of a predetermined masson said collector plate; detector circuit means connected to saidcollector plate; means to introduce a plurality of sample streams to beanalyzed into said ionization chamber in sequence comprising a pluralityof first conduit means having respective first control valves therein,second conduit means communicating at one end with said irst conduitmeans downstream from said control valves, third conduit meanscommunicating with the second end of said second conduit means to ventiluid, fourth conduit means having a iirst restriction therein, one endof said fourth conduit means communicating with the second end of saidsecond conduit means, a irst vacuum pump having the inlet thereofcommunicating with the second end of said fourth conduit means, fthconduit means having a second restriction therein communicating betweenthe second end of said fourth conduit means and the ionization chamberin said envelope, and a second Vacuum pump having the inlet thereofcommunicating with the interior of said envelope, and timing means toactuate said means to introduce and said switching means so as to applysaid first source of potential to said grid for a predetermined time andthereafter to apply said second source of potential to said grid eachtime a different stream is passed to said ionization chamber.

References Cited in the le of this patent UNITED STATES PATENTS2,551,637 Robinson May 8, 1951 2,650,306 Robinson Aug. 25, 19532,652,495 Washburn Sept. 15, 1953 2,714,164 Riggle et al July 26, 19552,761,974 Burk et al. Sept. 4, 1956 2,792,542 Robinson May 14, 1957OTHER REFERENCES Lanneau: Development and Application of Process MonitorMass Spectrometers, Applied Mass Spectrometry, published by theInstitute of Petroleum, London, 1954, pages 197 -to 217.

